The linewidth of a laser is of prime importance for a certain number of applications. Mention may be made, for example, of coherent optical transmission systems with high transmission rates, for which a linewidth below 300 kHz is required when used in QAM (Quadrature Amplitude Modulation) mode. To generate a millimetre-wavelength or terahertz wave using the beating of two single-wavelength lasers, the linewidth of these lasers determines the linewidth of the millimetre-wavelength or terahertz wave.
Several solutions exist to reduce the linewidth of a single-wavelength laser source.
The first is of course to design a laser source so that the linewidth is minimized. But it is not always possible to obtain the requisite width in this way.
Another solution is based on external optical back-projection, as illustrated in FIG. 1a. The light 10 emitted by a single-wavelength laser source 1 is reflected by an external reflector 2 and re-injected into the laser source. Under certain phase conditions, the linewidth of the laser can be reduced. But the external reflector generally creates strong resonances in the phase noise spectrum and in the optical emission spectrum of the laser as illustrated in FIG. 1b. However, these resonances, which correspond to those of the external cavity formed by the external mirror 2 and the exit face 11 of the laser source, are undesirable for a large number of applications.
A third solution consists in using random back-projection induced by Rayleigh scattering produced in an optical fibre. Rayleigh scattering is created partly by dopant ions in a single-mode optical fibre. It has been demonstrated that back-projection of Rayleigh type can reduce the linewidth by a factor of 106, for a wavelength at 1550 nm for example. But this back-projection of Rayleigh type is very weak. The mean of the power reflected by the Rayleigh scattering produced in a section of fibre of 1 km is in the order of 3×10−5, which necessitates the use of very long optical fibres, of a length above 1 km at least, to allow a significant reduction in linewidth to be obtained. As a result the laser device which incorporates the laser source and an optical fibre of such a length is very bulky. Furthermore, temperature variations and vibrations induce fluctuations in the Rayleigh scattering, which translate into undesirable fluctuations in the frequency of the laser.
Furthermore, all these solutions apply to single-wavelength sources only.
Consequently, there remains to date a need for a laser device simultaneously satisfying all the aforementioned requirements in terms of linewidth, bulkiness and frequency stability, and applicable to single- or multi-wavelength laser sources.